Communications systems for transferring digital data between two points are being used with increasing frequency, most commonly to allow one computer to access another computer. Although the cost of such systems has markedly decreased, the cost is nevertheless substantial enough to prevent widespread use of such systems in the home. However, widespread home use of such systems would allow users to automatically perform a wide variety of transactions. For example, individuals could perform most of their banking from the home by accessing a bank computer to transfer money from one account to another. Similarly, individuals could direct a bank computer to pay their bills from an account. Although the advantages of widespread residential data communication systems would be many, the relatively high cost of such systems has precluded such use.
Another problem with conventional data communication systems stems from the requirements of signals which are allowed to be transmitted over the telephone lines. Telephone companies generally require that the signal transmitted over the telephone lines be within a certain bandwidth and amplitude so that it does not interfere with signals on other lines. These telephone company requirements have limited, to some degree, the options which one can use to transmit data over telephone lines. The most common data communication technique is frequency shift keying (FSK), by which each of two binary states are represented by a sine wave of a respective frequency. However, shifting from one frequency to another can produced spurious signals outside of the allowable bandwidth.
Another common technique for transferring digital data over telephone lines utilizes conventional two-tone generators which are driven by conventional sixteen-button TOUCH-TONE keypads. A serious limitation of this data communication technique is that, since only sixteen discrete characters can be selected, the keypad is capable of sending only numeric data as opposed to alpha numeric data.